1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particular to a lateral-electric-field liquid crystal display device which exhibits excellent viewing angle characteristic and brightness characteristic.
2. Background Art
A liquid crystal display device includes a TFT substrate on which pixel electrodes, thin film transistors (TFT) and the like are formed in a matrix array and a counter substrate which faces the TFT substrate in an opposed manner and forms color filters or the like thereon at places corresponding to the pixel electrodes, and liquid crystal is sandwiched between the TFT substrate and the counter substrate. Further, an image is formed by controlling transmissivity of light for every pixel by driving liquid crystal molecules.
Viewing angle characteristic is important in the liquid crystal display device. The viewing angle characteristic is a phenomenon that brightness is changed or chromaticity is changed between a case where a screen is observed from a front side and a case where the screen is observed in the oblique direction. With respect to the viewing angle characteristic, an IPS (In Plane Switching) liquid crystal display device which operates liquid crystal molecules using an electric field which contains electric-field components in the horizontal direction exhibits excellent viewing angle characteristic.
Although the IPS liquid crystal display device exhibits the excellent viewing angle characteristic compared to a liquid crystal display device which adopts other liquid crystal driving method, the viewing angle characteristic of the IPS liquid crystal display device is less than optimal. For example, between a case where a screen is observed from one direction and a case where the screen is observed from other direction, the tone of color is delicately changed. Although a change of the tone of color is smaller than a change of the tone of color observed in a liquid crystal display device which adopts other liquid crystal drive method, there still exists a room for improvement with respect to the tone of color of the IPS liquid crystal display device. JP-A-2005-346064 (corresponding US patent application US2005/0264744 A1) (patent document 1) discloses the constitution where the directivity of viewing angle characteristic is improved by forming a pixel in a trapezoidal shape and by providing a region where the direction of a pixel electrode and the direction of a common electrode differ from each other in one pixel.
In the IPS liquid crystal display device disclosed in patent document 1, the pixel electrodes and the common electrodes are formed on the substantially same plane on a TFT substrate. Accordingly, transmissivity of light from a backlight on the TFT substrate, that is, brightness of screen becomes important.
An IPS liquid crystal display device disclosed in JP-A-2004-361949 (corresponding U.S. Pat. No. 7,009,206) (patent document 2) adopts the constitution where trapezoidal pixels are arranged in a row by changing the direction of a trapezoidal shape also in the lateral direction. In patent document 2, the pixels are defined by video signal lines and bent scanning lines. The scanning lines are bent to allow the scanning lines to conform with the trapezoidal pixels. Further, in patent document 2, a capacitive line extends at the center of the pixel. The technique disclosed in patent document 2 is substantially equal to the technique disclosed in patent document 1 with respect to a point that the pixel electrodes and the common electrodes are formed on the substantially same plane on a TFT substrate. Further, the technique disclosed in patent document 2 is disadvantageous in transmissivity with respect to a point that the capacitive line extends at the center of the pixel.
As the constitution which increases transmissivity in an IPS liquid crystal display device, the constitution disclosed in JP-A-2003-21845 (corresponding U.S. Pat. No. 6,747,712) (patent document 3) is named. In this constitution, the pixel electrodes and the common electrodes are formed in different layers, and the common electrode is formed of a planar electrode. The pixel electrode is formed in a comb-teeth shape with a closed distal end, and an image is formed by controlling liquid crystal molecules using an electric field generated by the potential difference between the comb-teeth-shaped electrode and the common electrode. With the use of a transparent electrode as the pixel electrode and the common electrode, transmissivity is largely increased.
Patent document 3 discloses the constitution where the pixels are defined by capacitive lines (common lines) and video signal lines, and a scanning line is arranged at the center of the pixel. Also in such an IPS liquid crystal display device, due to the presence of the capacitive lines for forming capacitances, transmissivity is lowered.